A quantum-key distribution system includes a transmitter, a receiver, and an optical fiber link that connects the foregoing. The transmitter transmits a stream of single photons to the receiver via the optical fiber link (a quantum communication channel) that is a communication channel of an optical fiber. Thereafter, the transmitter and the receiver exchange control information with each other, and thereby the transmitter and the receiver share an encryption key between them. This technology is implemented by a technology generally referred to as quantum key distribution (QKD).
To share the encryption key between the transmitter and the receiver by the quantum key distribution, there is the need to perform key distillation processing on each of the transmitter and the receiver. The key distillation processing is composed of shifting processing, error correction processing, and privacy amplification processing. By this key distillation processing, the transmitter and the receiver share the encryption key. The shared encryption key is used, as a key of one-time pad, when encrypted data communication is performed between the transmitter and the receiver or between applications coupled to the respective apparatuses. In the encrypted data communication by the encryption key of a one-time pad, by information theory, it has been assured that no eavesdropper having any knowledge can decipher it.
In the quantum key distribution, the photon used to share the encryption key has the uncertainty principle that is one of the fundamental principles in quantum mechanics in which the physical state thereof changes by being observed. By this principle, when an eavesdropper observed, on a quantum communication channel, photons including information on an encryption key transmitted by the transmitter, the physical state of the photons is changed and thereby the receiver that received the photons can tell that the photons have been observed by the eavesdropper. At that time, the change in the physical state of the photons appears as a quantum bit error rate (QBER) of the link (quantum communication channel) between the transmitter and the receiver. When an eavesdropper attempts to observe photons, the receiver and the transmitter can tell the presence of the eavesdropper because the physical state of the photons is changed and the QBER is increased.
The generation amount of encryption keys shared per unit time is referred to as a secure key rate (an encryption-key generation rate). As the number of encryption keys being available increases, encrypted data communication becomes faster and safer, and thus a higher secure key rate can implement a high-performance quantum-key distribution system. In a quantum-key distribution system, at the time of starting up the apparatus or as part of maintenance work at the time of recovering from a fault and the like, there is the need to set adjustment items in respective pieces of processing, in encryption-key generation operation of the transmitter and the receiver, to improve the secure key rate. As such a quantum-key distribution system, there is one that makes adjustments in the operation, out of the encryption-key generation operation, of transmitting photons to a receiver from a transmitter via a quantum communication channel.
As for the information necessary to calculate the secure key rate, however, the accurate value thereof is not known until the entire processing in the encryption-key generation operation is completed. At a phase in the middle of the encryption-key generation operation, it is not easy to instantaneously determine what effect the current settings have on the secure key rate that is finally obtained. In the above-described system, although the adjustments in the operation of transmitting photons are made, it is not sufficient to improve the secure key rate because it does not set the adjustment items in other processing of the encryption-key generation operation. It is therefore necessary to determine what adjustment items ought to be changed by trial and error and by reference to the experience of past adjustment work, and thus the maintenance work takes time and the cost for system maintenance increases.